Pili are filamentous structures on the surface of many bacterial cells. Pili are involved in the process of infection by a number of pathogens and also are important to normal bacterial physiology. However, a variety of conditions affect the extent of piliation. In this project, the control of bacterial piliation will be investigated by taking advantage of a recent discovery that synthesis of pilin, the subunit polypeptide of pili, can be induced by dye-sensitized photoxidation. A species of Arthrobacter which is used in these experiments, is lysed by SDS and thus allows analysis of only surface proteins. Within minutes after cells are exposed to light in the presence of a dye, newly synthesized pilin appears on the cell surface. When cells are transferred to the dark, an extensive array of pili appears. Synthesis of pilin is measured by quantitative radioautography after resolution of the 21,000-dalton polypeptide by slab gel electrophoresis in the presence of SDS. To determine if the induction results from oxidation of pili, these structures will be purified and changes in the composition will be analyzed after they are incubated under photooxidative conditions. The involvement of singlet oxygen in the induction process will be examined by adding scavengers of this excited species. The kinetics of synthesis of pilin will be examined during the induction phase and after the cells are placed in the dark to remove the stimulatory conditions. The experiments are designed to gain information on the mechanisms that control synthesis of pilin and, as a consequence, piliation. Subsequent experiments will investigate the site and mechanism of assembly of pili.